In the past, there have been proposed communication systems in which masters serving as coordinators and slaves serving as end devices perform wireless communication with each other. In these communication systems, a communication frame F100 including a beacon signal as its head (shown in FIG. 22).
The communication frame F100 defines virtual communication time slots defined by thirty-one time slots. Thus, each communication terminal on a network is allowed to send and receive signals at a predetermined timing rather than any timing. The thirty-one time slots are one beacon slot TS101 provided as a head of the communication frame F100, and thirty data slots TS102 provided subsequent to the beacon slot TS101. The beacon slot TS101 is a time slot for sending the beacon signal in a downstream direction from the master to the slave. The beacon signal is used for making communication synchronization between the master and the slave. Each data time slot TS102 is a time slot for sending a signal in an upstream direction from the slave to the master. When receiving an upstream signal in a time slot, the master sends a downstream signal in the same time slot, if necessary.
The master sends the beacon signal by use of the beacon slot TS101. The slave makes synchronization with the master based on a time point of receiving the beacon signal, thereby communicating with the master by use of the data slot TS102 of the communication frame F100.
Next, an operation in a case where the master is activated prior to the slave is described. The master activated earlier sends the beacon signal intermittently. The slave activated later performs a continuous receiving of continuing a receiving operation throughout a certain time period, initially. The time period for the continuous receiving is set to be longer than one cycle of the communication frame F100 (i.e., a cycle of sending the beacon signal). Therefore, when the slave is activated with the master being already in operation, the slave can receive the beacon signal sent from the master.
Next, an operation in a case where the slave is activated prior to the master is described. At a time point of activating the slave, the master is not activated yet, and therefore the master does not send the beacon signal. When failing to receive the beacon signal, the slave performs an intermittently receiving operation at an intermittently receiving interval based on time measured by a clock built in the slave. When activated, the master sends an activation signal. A time period for sending the activation signal is set to be longer than the intermittently receiving interval, and thus the slave can receive the activation signal. When receiving the activation signal, the slave extends the receiving operation to receive the beacon signal which is sent from the master subsequent to the activation signal (e.g., see JP 2011-101276 A). Accordingly, even if the slave is activated in a situation where the master is not activated yet, the slave can receive the beacon signal to make the synchronization, and therefore can establish communication by use of the communication frame F100.
According to the above background art, in a case where the slave is activated prior to the master, the slave fails to receive the beacon signal immediately after activation. Thus, the slave performs the intermittently receiving operation at the intermittently receiving interval based on the time measured by the clock built in the slave. When succeeding in receiving the beacon signal, the slave establishes communication by use of the communication frame F100. However, there is a great difference in the communication scheme between the aforementioned intermittently receiving operation without using the communication frame and the communication by use of the communication frame. Accordingly, the slave is required to conform with such two greatly different communication schemes, and this may lead to an increase in a load necessary for a communication process.